The present invention relates to a semiconductor light emitting element, and more particularly to a light emitting element suitable for use as an infrared light source for an automatic focusing (AF) mechanism assembled in a camera.
Recently, GaAlAs infrared light emitting diodes are widely used as a light source of infrared rays for a camera's automatic focusing mechanism. Infrared rays emitted therefrom are passed through a collimator lens into a parallel beam, emitted toward a subject, and then reflected from the subject. The light reflected from the subject is sensed by a light receiving element, and a distance to the subject is measured in accordance with a trigonometrical survey method.
The above-mentioned infrared light emitting element was disclosed in Japanese Patent Laid-open (Kokai) No. 63-169775 (1988), for instance, FIG. 3 is a cross-sectional view of this prior-art element, in which an n-type current restriction layer 5 is formed on a p-type GaAs semiconductor substrate 6 by a liquid phase epitaxial growth method (referred to as LPE method, hereinafter), and further Zn atoms are diffused into part of the n-type current restriction layer 5 to form p-type range by selective diffusion technique. Further, a p-type GaAlAs cladding layer 4, a p-type GaAlAs active layer 3, and an n-type GaAlAs cladding layer 2 are formed in sequence on the n-type current restriction layer 5 a in stacking manner for improvement of the productivity. Further, an ohmic electrode 1 of an AuGe alloy is formed on the surface of the n-type GaAlAs clad layer 2 and another ohmic electrode 7 of an AuBe alloy is formed on the surface of the p-type semiconductor substrate 6.
In this element, a GaAlAs double heterojunction (referred to as DH structure, hereinafter) is adopted to increase the emitted light power; the light emitting portion 9 is narrowed into a point light source so that the emitted light can reach a farthest possible position to increase the measurable distance; and the current restriction layer (region) 5 is formed inside the pellet to prevent other regions from emitting light. Further, since it is necessary to determine the wavelength of the emitted light reflected from the subject and received by the element, to be approximately 860 nm, in order to increase the sensitivity to infrared rays emitted to a subject, the mixed crystal ratio of AlAs in the p-type GaAlAs active layer 3 is set to 0.03.
Light emitting elements thus constructed as described above, have been widely used at present, and the quantity of production of these elements is increasing year by year. However, when a single light emitting element is used as a light source for the AF mechanism and therefore light is emitted from a single point, there exist some cases where a subject is taken in out-of-focus status. In more detail, in the AF camera in general, a subject is brought into focus at the central portion within a finder. Therefore, where a portrait is taken, if a person is not located at the central portion of the finder, an object behind the person is brought into focus. The rate at which the above-mentioned out-of-focus occurs is about 20% in the case where the camera emits AF light from a single point.
To overcome the above-mentioned problem, a method of mounting three light sources to emit light from three different points has been proposed. In this method, however, precision as high as .+-.50 .mu.m or less is required for the distance intervals between the two light sources. This is because when three light emitting elements are activated in sequence in response to a pulse signal to emit beams through lenses toward a subject and the light reflected from the subject is received by a light receiving element, if each distance between two light emitting elements is offset from a predetermined value, three reflected rays are not focused on the light receiving element simultaneously, so that it is impossible to bring the subject into focus.
In the conventional method, however, the distance precision between two light emitting elements has been about 200 .mu.m at the most, because these light emitting elements are mounted on each camera frame by hand. In addition, although each light emitting element is cut away from a semiconductor wafer by dicing, since the positional offset (dispersion) in the dicing process is generally .+-.15 .mu.m, even if the mounting precision of the elements on a camera frame is improved, it is impossible to prevent the distance intervals between the two light emitting elements from being offset from a predetermined value. Therefore, the method of mounting three light emitting elements on a camera for prevention of out-of-focus problem lies only at a stage of an idea, without being so far realized.